The present invention relates generally to laser trimming and more specifically to laser trimming above a dielectrically isolated single crystalline region.
Laser trimming of thin-film resistors is used extensively to produce improved accuracy in analog integrated circuit technology. In integrating laser trimming into dielectrically isolated circuit technology a difficulty peculiar to dielectric isolation has been identified. Trimming is generally accomplished by use of an infrared laser for improved control. Silicon is nearly transparent at the wavelengths used; this results in laser energy penetrating to the bottom of the dielectrically isolated island, reflecting back and transferring some of the reflected energy back to the resistor. The result is poor control due to interference effects. These effects are variable due to changes in dielectrically isolated island depth and proper control over trim energy hence becomes very difficult.
An existing technique addresses the problem by simply placing the thin-film resistor over the polycrystalline silicon used to support the dielectrically isolated regions. The polysilicon is much thicker than the single-crystal islands (typically 10 mils vs. 1 mil) and energy scattering off the grain boundaries soon dissipates the laser beam. The resulting lack of reflection and interference produces enhanced controllability. The resistor is deposited over a polycrystalline surface which is not perfectly flat but in fact possesses considerable relief at the grain boundaries. In addition, large polycrystalline areas tend to "dish out" which complicates laser focussing and reduces photoresist definition, resulting in poor control over resistor geometries. There is also concern over the validity of thin film over polycrystalline systems for high-reliability applications.